1. Field of the Invention
The present invention relates to a method of and an apparatus for the recovery of metallic aluminum from furnace dross containing only a minor amount of aluminum. More particularly, the present invention relates to the recovery from furnace dross that includes a mechanical mixture of metallic aluminum, solidified salts, various oxides of aluminum and magnesium, various carbides, spinel, and other materials, of a component containing a relatively high metallic aluminum content, to minimize the amount of flux that must be disposed of after recovery of the metallic aluminum.
2. Description of the Related Art
The melting of aluminum-containing scrap materials, such as used aluminum alloy beverage cans and other products containing metallic aluminum, is often carried out in a reverberatory furnace. That type of furnace typically has an interior hearth connected to an external well, into which a charge of metal-containing materials, such as scrap aluminum beverage cans, is introduced. The charge is melted by supplying heated air to the space between the molten metal present in the hearth and the low roof of the furnace and circulating the heated metal from the interior to the exterior well. The heat source for heating the air is usually provided by the combustion of a fuel, such as natural gas, or the like. The metallic and other materials included in the scrap charge, including fluxes, are maintained in the hearth in a molten state for a predetermined time.
During the melting process the added salt flux forms an increasingly viscous, black mass of furnace dross that includes entrained oxides, aluminum-magnesium spinels, dirt, and other materials introduced with the scrap. The viscous dross layer impedes the assimilation into the melt of additional scrap material, and thereby limits the quantity of scrap material that can be processed. In order to permit additional scrap to be added to the melt, the dross must be skimmed off the surface of the melt, after which additional scrap material can be added to provide the quantity of recovered molten metallic aluminum needed for subsequent use.
The viscous dross that is skimmed from the surface of the melt is a mixture of a residual, minor amount of metallic aluminum, along with various metallic salts and other waste materials. Upon cooling to ambient temperature the dross solidifies to form a black mass that can contain up to about 20% by weight metallic aluminum. The amount of metallic aluminum present in the dross is dependent upon furnace operating conditions, but typically it is in the range of about 10% by weight.
Recovery of the metallic aluminum contained in the dross has heretofore been effected by crushing, screening, re-crushing, and re-screening the dross (see FIG. 1) in order to reduce to fine particles the more friable salt flux so that it can be readily separated from the larger particles of the less friable metallic constituents. The latter include metallic skulls and an aluminum-containing portion that includes approximately 50% metallic aluminum. The aluminum-containing portion is fed into a rotary furnace, along with up to several times its weight of a salt flux, to enable the release and coalescence of the aluminum metal that is present.
After the removal of the metallic aluminum, there remains a resulting salt cake, based in part upon the flux carried over from the reverberatory furnace and also in part upon the flux carried over from the rotary furnace, and that salt cake must be disposed of. However, land disposal of rotary furnace salt cake residue, which contains less than about 5% aluminum, is becoming increasingly expensive, and is also the subject of environmental regulations.
It is an object of the present invention to recover from the reverberatory furnace dross as much of the metallic aluminum as reasonably possible, to minimize the amount of material to be processed in a rotary furnace, and also to minimize the amount of salt cake residue requiring disposal.